JP2009263165A - Reactive ceramic and method for producing the same, and hydrogen production method and hydrogen production apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide reactive ceramics promoting an oxygen releasing reaction in air having high oxygen partial pressure and having high reactivity and high temperature stability in a two-stage water decomposition reaction using concentrated solar energy, and continuously producing hydrogen from water and solar energy by repeating a reduction reaction (oxygen releasing reaction) and hydrolysis (hydrogen generating reaction), and to provide a method for producing the same, a hydrogen production method and a hydrogen production apparatus. <P>SOLUTION: The ceramics comprise a solid solution containing cerium and zirconium, in which both of cubic and tetragonal crystal systems are present. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a reactive ceramic used for hydrogen production by concentrated solar energy, a production method thereof, a hydrogen production method, and a hydrogen production apparatus.

  In recent years, as a solution to problems such as depletion of fossil fuels and global warming due to an increase in carbon dioxide, development of clean renewable energy that does not emit carbon dioxide in place of fossil fuels has become increasingly important. Solar energy, which is one of the renewable energies, does not have to worry about exhaustion and can contribute to the reduction of greenhouse gases. In recent years, fuel cells have begun to spread and are expected to be a driving force for the hydrogen energy society, but most of the currently produced hydrogen is made from fossil fuels, which is a problem in terms of fundamental fossil fuel reduction. There is. In such a world situation, a form in which primary energy is obtained from solar energy and secondary energy is supported by hydrogen is one of the ideal clean energy systems, and its establishment is urgent.

  Therefore, if sunlight is condensed and hydrogen or methanol can be produced and stored as storable chemical energy using the energy of the collected sunlight, the usefulness as an energy resource increases.

  By the way, there is a region in the world where the sun falls abundantly throughout the year, called Sunbelt regions such as the southern United States, the Mediterranean Sea and Australia. In such areas, where most of the undeveloped deserts are located, it has been proposed to produce concentrated solar power using abundant solar energy and to produce hydrogen and methanol using solar energy. Is underway.

Therefore, hydrogen production by a two-stage water splitting reaction using reactive ceramics has been proposed as one method of converting condensed solar energy into chemical energy (see Non-Patent Documents 1 to 3). As shown in FIG. 9, this two-stage water splitting reaction is composed of two stages: a production process of reduced reactive ceramics (oxygen release reaction) and a water splitting process (hydrogen generation reaction) using reduced ceramics.
Oxygen release reaction MO _ox = MO _red + 1 / 2O ₂ (1)
Hydrogen generation reaction MO _red + H ₂ O = MO _ox + H ₂ (2)

Since the generation process (oxygen release reaction) of the reduced reactive ceramics is an endothermic reaction, it can absorb the solar energy of concentrated solar heat. As reactive ceramics used in the two-stage water splitting reaction, ferrite (see Non-Patent Documents 4 to 5), ceria (see Non-Patent Documents 6 to 7), and the like have been studied so far. The inventors of the present application are considering the practical application of a system that continuously produces hydrogen by a rotary solar reactor using reactive ceramics (see Non-Patent Document 8). As the reactive ceramics used in this rotary solar reactor, the oxygen release reaction can proceed in air with a high oxygen partial pressure, and can cope with a high energy flux (1000-3000 kW / m ² ) in the oxygen release process. A material having both high temperature stability and high reactivity is desired. Until now, it has been clarified that in a two-stage water splitting reaction using nickel ferrite, an oxygen releasing reaction proceeds in air (see Non-Patent Document 4).
Tamaura, Y., 2003: “Solar Fuel Production by Conversion of Concentrated Solar-Heat to Chemical Energy”, Eco-Engineering 15 (3), 109-119. Steinfeld, “A. Solar thermochemical production of hydrogen-a review.” Solar Energy 78, 603-615 (2005) Tamaura Hiroshi, Kaneko Hiroshi, Japanese Patent Application 2006-274864, “Hydrogen Production Method, Hydrogen Production Equipment and Metal Oxides”, (2006) Tamaura et al .: “Oxygen-releasing step of ZnFe2O4 / (ZnO + Fe3O4) -system in air using concentrated solar energy for solar hydrogen production”, Solar Energy, 78 (5), 616-622, 2005. Yuki Naganuma, Hiroshi Kaneko, Hideyuki Ishihara, Shunpei Taku, Shuhei Imaeda, Hiroaki Fukuzumi, Noriko Hasegawa, Hiroshi Tamaura, “Solar Hydrogen Production Using Ni-Ferrite—Use in Rotary Reactors for Practical Use” Proceedings of the 2006 Japan Solar Energy Society and Japan Wind Energy Association Joint Presentation Meeting, (2006), 417-419. H. Kaneko, T. Miura, H. Ishihara, S. Taku, T. Yokoyama, H. Nakajima and Y. Tamaura, “Reactive ceramics of CeO2-MOx (M = Mn, Fe, Ni, Cu) for H2generation by two -step water splitting using concentrated solar thermal energy ”, Energy (2007), 32 (5), 656-663. S. Taku, H. Kaneko and Y. Tamaura, “Two-step water splitting reaction with Ce-Zr oxide system for solar hydrogen production”, Eco-Engineering (2007), 19 (4), 255-262. H. Kaneko, T. Miura, A. Fuse, H. Ishihara, S. Taku, H. Fukuzumi, Y. Naganuma and Y. Tamaura, “Rotary-Type Solar Reactor for Solar Hydrogen Production with Two-step Water Splitting Process” , Energy & Fuels (2007), 21, 2287-2293.

However, in the two-stage water splitting reaction using nickel ferrite, the oxygen releasing reaction is performed near the melting point (1500 ° C or higher), so melting and sintering occur, and the two-stage water splitting reaction proceeds continuously. It was difficult. On the other hand, the conventional cubic type ceria-based reactive ceramic CeO ₂ —MOx (M = Mn, Fe, Ni, Cu) has an advantage that it has high temperature stability and the hydrogen generation reaction proceeds at a low temperature around 500 ° C. However, the progress of the oxygen release reaction in the air having a high oxygen partial pressure has not been achieved.

  Therefore, the problem of the present invention is that in a two-stage water splitting reaction using concentrated solar heat, oxygen release reaction progresses in air with a high oxygen partial pressure, high reactivity and high temperature stability, and a reduction reaction ( A reactive ceramic capable of continuously producing hydrogen from water and solar energy by repeating an oxygen release reaction) and a water splitting reaction (hydrogen generation reaction), a method for producing the same, a hydrogen production method, and a hydrogen production apparatus It is to provide.

The present inventors consider that conventional cubic-type ceria-based reactive ceramics proceed only through the redox reaction of Ce atoms, and thus exceed the upper limit at which ZrO ₂ can be substituted into the fluorite lattice of CeO _2. Oxygen deficiency suggested that CeO ₂ -ZrO ₂ -based reactive ceramics having both cubic and tetragonal crystal systems are accompanied by an interaction between these crystal systems. It was found that the oxygen release reaction proceeds in the air due to the formation.

  Therefore, in order to solve the above-mentioned problem, the reactive ceramic of the present invention is made of a solid solution containing cerium and zirconium based on the above knowledge, and is characterized in that a cubic and tetragonal crystal system coexist.

  This reactive ceramic is made of a solid solution containing cerium and zirconium, and coexisting with a cubic and tetragonal crystal system, it is heated to 1400-1800 ° C. in the atmosphere and reduced to release oxygen. It is possible to produce hydrogen by repeatedly performing a reaction and a two-stage water splitting reaction in which a reduced reactive ceramic is reacted with water at 300 to 1200 ° C. to generate hydrogen by an oxidation reaction. it can.

  The reactive ceramics of the invention according to claim 2 is characterized in that the cerium / zirconium molar ratio is 90/10 to 20/80.

  In this reactive ceramic, the molar ratio of cerium / zirconium is preferably 90/10 to 20/80, and the molar ratio of cerium / zirconium is preferably 80/20 to 50/50. preferable.

  According to a fourth aspect of the present invention, there is provided a method for producing a reactive ceramic comprising: a step of mixing a nitrate aqueous solution containing cerium nitrate and zirconium nitrate with an oxalic acid aqueous solution to form a precipitate; and a suspension containing the precipitate. A step of adjusting the pH of the liquid to 0 to 7 and a step of baking the precipitate.

  In this method for producing reactive ceramics, a nitrate aqueous solution containing cerium nitrate and zirconium nitrate, a step of generating a precipitate, a step of adjusting the pH of the suspension containing the precipitate to 0-7, By the step of firing the precipitate, a reactive ceramic made of a solid solution containing cerium and zirconium and having a cubic and tetragonal crystal system coexisting can be obtained.

  A method for producing hydrogen according to a fifth aspect of the present invention is a method for producing hydrogen using the reactive ceramics, wherein the reactive ceramics is irradiated with concentrated solar energy and 1400 in the air atmosphere. Heating to 1800 ° C. to reduce the reactive ceramics to release oxygen, and reacting the reduced reactive ceramics with water at 300 to 1200 ° C. to oxidize the reactive ceramics. It is characterized by repeatedly performing a two-stage water splitting reaction in which hydrogen generation reaction for generating hydrogen is performed.

  In this hydrogen production method, the reactive ceramic is irradiated with concentrated solar energy and heated to 1400 to 1800 ° C. in the air atmosphere to reduce the reactive ceramic to release oxygen, and the reduction The condensed reactive ceramics are reacted with water at 300 to 1200 ° C. to oxidize the reactive ceramics to generate hydrogen to repeatedly generate hydrogen, thereby performing a two-stage water splitting reaction. Hydrogen can be efficiently produced using energy.

The method for producing hydrogen according to claim 6 is characterized in that an energy flux of the concentrated solar energy is 1000 to 3000 kW / m ² .

In this hydrogen production method, hydrogen can be efficiently produced using concentrated solar energy with an energy flux of 1000 to 3000 kW / m ² .

  A hydrogen production apparatus according to a seventh aspect of the present invention is a two-stage water splitting reaction section provided with the reactive ceramics, a solar power supply section for supplying solar energy to the two-stage water splitting reaction section, A water supply part for supplying water to the two-stage water splitting reaction part, and in the two-stage water splitting reaction part, solar energy is supplied from the solar power supply part to cause the reactive ceramics to be in the atmosphere. Heating to 1400 to 1800 ° C. to reduce the reactive ceramics to produce oxygen, and further reacting the reduced reactive ceramics with water at 300 to 1200 ° C. to reduce the reduced reactive ceramics It is characterized in that hydrogen is produced by repeating a two-stage water splitting reaction that generates hydrogen by oxidation.

  In this hydrogen production apparatus, in a two-stage water splitting reaction section, solar energy is supplied from the solar power supply section and the reactive ceramic is heated to 1400 to 1800 ° C. in an air atmosphere to Reduction is performed to generate oxygen, and further, a two-stage water decomposition reaction is repeated in which the reduced reactive ceramic is reacted with water at 300 to 1200 ° C. to oxidize the reduced reactive ceramic and generate hydrogen. Thus, hydrogen can be efficiently produced using concentrated solar energy.

  The reactive ceramic of the present invention has a progress of oxygen release reaction in air with a high oxygen partial pressure, high reactivity, and high temperature stability in a two-stage water splitting reaction using concentrated solar heat. By repeating the oxygen releasing reaction) and the water splitting reaction (hydrogen generating reaction), it is possible to efficiently produce hydrogen continuously from water and solar energy.

  Moreover, according to the method for producing reactive ceramics of the present invention, it is possible to obtain reactive ceramics made of a solid solution containing cerium and zirconium and in which cubic and tetragonal crystal systems coexist.

  Furthermore, according to the hydrogen production method of the present invention, the oxygen release reaction in which the reactive ceramic is reduced to release oxygen, and the reactive ceramic is reacted with water in the atmosphere to oxidize the reactive ceramic. By repeatedly performing the two-stage water splitting reaction in which hydrogen is generated to generate hydrogen, hydrogen can be efficiently produced using concentrated solar energy.

  Furthermore, according to the hydrogen production apparatus of the present invention, the reactive ceramics is heated in the atmospheric air by using the concentrated solar energy, and the reactive ceramics are reduced to generate oxygen and further reduced. It is possible to efficiently produce hydrogen using concentrated solar energy by repeating the two-stage water splitting reaction in which the reactive ceramics are reacted with water in the atmosphere to generate hydrogen.

Hereinafter, the reactive ceramic of the present invention, a method for producing the same, a hydrogen production method, and a hydrogen production apparatus will be described in detail.
The reactive ceramic of the present invention is made of a solid solution containing cerium and zirconium. Cerium and zirconium are in solid solution at an upper limit of 10-30 mol% [1000 ° C. (firing temperature) -1500 ° C. (oxygen releasing reaction temperature)) at which ZrO ₂ can be substituted in the fluorite lattice of CeO ₂ . Is. The reactive ceramic of the present invention has a crystal form in which a cubic crystal system and a tetragonal crystal system coexist. The content ratio of the cubic and tetragonal crystal system varies depending on the phase transition depending on the temperature of the reactive ceramic. That is, this reactive ceramic undergoes a cubic-tetragonal phase transition at around 1500 ° C., and the oxygen release reaction proceeds in the air due to the generation of oxygen vacancies suggested to involve the interaction between these crystal systems. In addition, since ZrO ₂ (mp about 2700 ° C.) has a high melting point of CeO ₂ (mp about 2600 ° C.) or higher, transition metal (Mn, Fe, Ni, Cu) oxides conventionally dissolved in CeO ₂ Thus, there is an advantage that sintering and melting do not proceed even if the fluorite lattice is not replaced. For this reason, the reactive ceramic of the present invention undergoes an oxygen releasing reaction at around 1500 ° C., and the hydrogen generation reaction proceeds rapidly at a low temperature around 500 ° C., and has high temperature stability and high reactivity. It is useful for continuous hydrogen production by two-stage water splitting reaction using light solar heat.

  In the reactive ceramic of the present invention, the content ratio of cerium and zirconium is preferably 90/10 to 20/80 in terms of Ce / Zr molar ratio, and more preferably 80/20 to in terms of Ce / Zr molar ratio. 50/50. In particular, reactive ceramics containing a Ce / Zr molar ratio of 60/40 are preferred because of the large amount of hydrogen produced.

  The production of the reactive ceramic of the present invention includes a step of preparing a mixed solution by mixing a nitrate aqueous solution containing cerium nitrate and zirconium nitrate with an oxalic acid aqueous solution, and adjusting the pH of the mixed solution to form a precipitate. It can carry out by the method of including the process of producing | generating and the process of baking the said precipitate. This method is a so-called oxalate coprecipitation method. By this method, it is possible to obtain a reactive ceramic which is composed of a solid solution containing cerium and zirconium and has a crystal form in which a cubic crystal system and a tetragonal crystal system coexist. On the other hand, depending on the so-called complex polymerization method used to obtain this type of solid solution, reactive ceramics composed of a solid solution containing cerium and zirconium and having a crystal form in which cubic and tetragonal crystal systems coexist Thus, a ceramic composed only of a tetragonal crystal system can be obtained.

  Cerium nitrates include cerium nitrate, ammonium cerium nitrate, diammonium nitrate, cerium nitrate cerium, cerium oxalate, cerium chloride, cerium bromide, cerium carbonate, cerium acetate, cerium fluoride, cerium boride, cerium iodide, phosphorus Cerium acid can be used, and zirconium nitrate includes zirconium nitrate, zirconium nitrate, zirconium chloride, zirconium oxychloride, zirconium fluoride, zirconium oxyphosphate, zirconium silicate, zirconium bromide, acetic acid. Zirconium, zirconium hydroxide, zirconium carbonate, or the like can be used. These cerium nitrate and zirconium nitrate are mixed with an oxalic acid aqueous solution at a ratio of the Ce / Zr molar ratio to prepare a mixed solution. At this time, it is preferable that the oxalic acid aqueous solution is mixed so that the ratio is at least equivalent to the total amount of cerium nitrate and zirconium nitrate.

  The pH of the mixed solution is adjusted to a range of 0 to 7, preferably a range of 2 to 3. This pH adjustment increases the yield.

  The firing of the precipitate is preferably performed at about 900 to 1500 ° C., particularly preferably 1000 to 1400 ° C., in order to obtain a solid solution. The firing of the precipitate is preferably performed by adjusting the temperature using an electric furnace or the like.

  By using the reactive ceramic of the present invention, hydrogen and oxygen can be produced by a two-stage water splitting reaction in which an oxygen releasing reaction and a hydrogen generating reaction are repeated.

The oxygen releasing reaction is a reaction in which the reactive ceramic of the present invention is irradiated with condensed sunlight energy and heated to reduce the reactive ceramic and release oxygen.
Oxygen release reaction Reactive ceramics _ox = Reactive ceramics _red + 1 / 2O ₂ (3)

The concentrated solar energy for irradiating the reactive ceramics preferably has an energy flux of 1000 to 3000 kW / m ² from the viewpoint of the heat resistance of the reactor, the reaction characteristics of the reactive ceramics, and the heat resistance. The concentrated solar energy needs to be absorbed by the oxygen release reaction (endothermic reaction) of the reactive ceramics.

In the oxygen releasing reaction, the temperature for heating the reactive ceramics is about 1400 to 1800 ° C, preferably about 1500 to 1600 ° C. The reactive ceramic of the present invention causes an oxygen releasing reaction at a lower temperature than conventional reactive ceramics of about 1400 to 1800 ° C. In this respect, ZrO ₂ (mp about 2700 ° C.) is CeO ₂ (mp Since the oxygen releasing reaction occurs at a temperature lower than about 2600 ° C., the oxygen releasing reaction is not stopped by sintering or melting, which is effective.

  The reaction atmosphere in the oxygen releasing reaction is preferably an oxygen partial pressure range of 0 to 20%, and may be an air atmosphere. In this respect, the reactive ceramic of the present invention is practically more effective than conventional reactive ceramics that require an oxygen releasing reaction in an inert atmosphere such as Ar.

The hydrogen generation reaction is a reaction in which reactive ceramics reduced by an oxygen release reaction are reacted with water in the atmosphere to oxidize the reactive ceramics to generate hydrogen.
Hydrogen generation reaction Reactive ceramics _red + H ₂ O = MO _ox + H ₂ (2)

In the hydrogen generation reaction, it is preferable that the water to be reacted with the reactive ceramic is added to the reactive ceramic at a ratio of at least an equivalent and reacted. At this time, the temperature which heats a reactive ceramic is about 300-1200 degreeC, Preferably it is 400-600 degreeC.
A method for producing hydrogen, wherein the two-stage water splitting reaction is repeated.

  As described above, hydrogen can be continuously produced from water and solar energy by repeating the reduction reaction (oxygen release reaction) and the water splitting reaction (hydrogen generation reaction).

  As an apparatus for producing hydrogen using the reactive ceramic of the present invention, a two-stage water splitting reaction section provided with the reactive ceramic of the present invention, and sunlight supplying solar energy to the two-stage water splitting reaction section And a water supply unit that supplies water to the two-stage water splitting reaction unit, wherein the reactive ceramics supplies solar energy from the solar power supply unit in the two-stage water splitting reaction unit. Is heated in an air atmosphere to reduce the reactive ceramics to produce oxygen, and further, the reduced reactive ceramics are reacted with water to oxidize the reduced reactive ceramics to generate hydrogen. An apparatus that repeats the generated two-stage water splitting reaction is useful.

  In this hydrogen production apparatus, the temperature at which solar energy is supplied to heat the reactive ceramics into the air atmosphere is preferably in the range of 1400 to 1800 ° C, particularly preferably 1500 to 1600 ° C. Moreover, the heating temperature at the time of making the reduced reactive ceramics react with water becomes like this. Preferably it is 300-1200 degreeC, Most preferably, it is the range of 400-600 degreeC.

A specific example of this hydrogen production apparatus is the apparatus shown in FIG.
This hydrogen production apparatus 1 has a structure in which a plurality of reaction cells 3 are arranged around the outer periphery of a rotary rotor 2 with the reactive ceramics 4 facing outward, and oxygen is released by the reactive ceramics. A discharge cell 5 and a hydrogen generation cell 6 for performing a hydrogen generation reaction with reactive ceramics are provided.

  As shown in FIG. 1B, the reaction cell 3 has a structure in which a reactive ceramic 4 having a circular arc cross section is attached to a stainless steel case 3c together with heat insulating materials 3a and 3b. The reactive ceramic 4 is disposed in the reaction cell 3 so that the upper surface of the reactive ceramic 4 faces the outside of the rotary rotor 2 when the reaction cell 3 is attached to the outer periphery of the rotary rotor 2.

  The oxygen release cell 5 includes a sunlight introduction quartz window 5a provided outside for introducing the condensed sunlight and irradiating the reaction cell 3, and an outer cylindrical body for passing the introduced condensed sunlight. 7 has a closed space surrounded by the quartz window supports 7a and 7b. And it has the oxygen outlet 5c which derives | leads-out oxygen produced | generated by oxygen release reaction, and the gas inlet 5d for introduce | transducing the carrier gas (for example, Ar) for deriving the produced | generated oxygen.

  The hydrogen generation cell 6 includes a quartz window 6a for introducing condensed sunlight for heating the reactive ceramics 4, a window 6b provided in the outer cylindrical body 7 for allowing the introduced condensed sunlight to pass through, A closed space surrounded by the quartz window supports 8a and 8b is formed. And it has the hydrogen outlet 6c which leads out the hydrogen produced | generated by hydrogen production reaction, and the water vapor inlet 6d for supplying water vapor | steam to the produced | generated reactive ceramics 4. FIG.

  In this hydrogen production apparatus 1, when the rotary rotor 2 is rotated in the direction of arrow A and the reaction cell 3 enters the oxygen release cell 5, the solar light is introduced from the sunlight introducing quartz window 5 a through the window 5 b. Light is irradiated to the reactive ceramics 4 of the reaction cell 3 and heated to 1400-1800 ° C. As a result, the reactive ceramic 4 causes an oxygen releasing reaction, and oxygen is released from the reactive ceramic 4. The released oxygen is led out from the oxygen outlet 5c by the carrier gas introduced from the gas inlet 5d.

  Next, the reaction cell 3 in which the reactive ceramic 4 reduced by the oxygen release reaction in the oxygen release cell 5 is disposed enters the hydrogen generation cell 6 as the rotary rotor 2 rotates. Then, the condensed sunlight is irradiated from the quartz window 6a through the window 6b to the reduced reactive ceramic 4 of the reaction cell 3, heated to 300 to 1200 ° C., and introduced from the water vapor inlet 6d. Reacts with water vapor to produce hydrogen. The produced hydrogen is taken out from the hydrogen outlet 6c.

  As described above, in this hydrogen production apparatus 1, as the rotary rotor 2 rotates, the reactive ceramic 4 repeats the oxygen release reaction in the oxygen release cell 5 and the hydrogen generation reaction in the hydrogen generation cell 6. It can be carried out. Thereby, hydrogen can be continuously and efficiently produced using the energy of the concentrated sunlight. In addition, oxygen generated by the oxygen releasing reaction can also be used effectively.

  Hereinafter, the present invention will be described in more detail by way of examples and comparative examples of the present invention, but the present invention is not limited to these examples.

Example 1
Samples in which Ce and Zr were dissolved in the molar ratios of Ce / Zr at 100/0, 80/20 and 60/40, respectively, were prepared by the oxalate coprecipitation method as follows.

  An aqueous ammonia solution was added to each of the mixed solutions in which an aqueous nitrate solution containing Ce ions and Zr ions at a molar ratio of Ce / Zr of 100/0, 80/20, and 60/40 and an aqueous oxalic acid solution were mixed, and pH 3. After adjusting to 2 and stirring for 24 hours, the precipitate was separated by centrifugation. Each of the obtained precipitates was dried at 80 ° C. for 24 hours and then fired in an electric furnace at 1000 ° C. for 1 hour to obtain a ceramic sample.

  About the obtained ceramic sample, the X-ray-diffraction pattern was measured using the X-ray-diffraction apparatus (The Rigaku Electric Co., Ltd. make, RINT2100; CuK alpha ray). The obtained X-ray diffraction pattern is shown in FIG.

From the X-ray diffraction pattern shown in FIG. 2, each of the samples solid-solved at Ce / Zr molar ratios of 100/0, 80/20, and 60/40 is cubic derived from the fluorite structure of CeO _2. The crystal system mainly includes CeO ₂ (Ce / Zr molar ratio: 100/0), only cubic crystals, Ce _0.8 Zr _0.2 O ₂ (Ce / Zr molar ratio: 80/20) Is a cubic crystal and a very small amount of tetragonal crystal, and Ce _0.6 Zr _0.4 O ₂ (Ce / Zr molar ratio: 60/40) is confirmed to contain two crystal systems of a cubic crystal and a tetragonal crystal. It was. It was also found that all diffraction peaks confirmed by the X-ray diffraction pattern were due to a solid solution of CeO ₂ and ZrO ₂ .

Next, using the experimental apparatus 31 for performing the oxygen release reaction and the hydrogen generation reaction shown in FIG. 3, the obtained ceramic sample was subjected to an oxygen release reaction.
The experimental apparatus 31 includes a reaction tube 32 made of a quartz tube having a reaction part 33 in which a platinum net 33a is disposed, an infrared image furnace 34 disposed outside the reaction tube 32, and a pipe line 35a to the reaction part 33. A water vapor generator 36 for supplying water vapor and an analysis unit 37 for analyzing the gas generated in the reaction unit 33 and led out through the pipe line 35b. A thermocouple 38 for measuring temperature is inserted in the reaction unit 33. The steam generator 36 includes a water storage unit 61 that stores water, a micropump 62 that supplies water from the water storage unit 61 to the steam generation unit 63, and water that is supplied to the steam generation unit 63 to heat the water to generate steam. An electric furnace 64 to be generated and gas introduction ports 65a and 65b for introducing gas are provided. The analysis unit 37 includes a primary cooling unit 71 that primarily cools the gas generated in the reaction unit 33 with cooling water, a secondary cooling unit 72 that further cools the gas primarily cooled in the primary cooling unit 71 with ice, A direct gas mass spectrometer 73 is connected to the secondary cooling unit 72 and directly introduces gas from the secondary cooling unit 72 to perform mass analysis. Further, 39a and 39b are conduit switching valves, and 40 is a gas inlet.

A ceramic sample is placed on the platinum net 33 in the reaction tube 32 of the experimental apparatus 1, and the ceramic sample S is heated by irradiating the infrared image furnace 34 with infrared rays as pseudo-condensed sunlight to perform an oxygen release reaction experiment. It was. In the experiment, the pipeline switching valve 39a is closed, the pipeline switching valve 39b is opened, and Ar gas or air is supplied from the gas inlet 40 to the reaction unit 33 to perform the oxygen release reaction of the reactive ceramic sample S. It was. At this time, while the temperature of the reactive ceramic sample S is measured by the thermocouple 38, the heating of the reactive ceramic sample S by infrared rays from the infrared image furnace 33 is performed so that the temperature rising rate becomes 200 ° C./min. Adjusted. The reactive ceramic sample S was subjected to an oxygen releasing reaction at 1500 ° C. for 5 minutes while supplying argon or air. The gas generated by the oxygen release reaction is led out to the analysis unit 7 through the pipe 5b, cooled by the primary cooling unit 71 and the secondary cooling unit 72, and then directly introduced into the gas mass spectrometer 73 to measure the amount of oxygen. did. Table 1 shows the oxygen release amount measured for each ceramic sample S (CeO ₂ , Ce _0.8 Zr _0.2 O ₂ , Ce _0.6 Zr _0.4 O ₂ ). As a result, CeO ₂ and Ce _0.8 Zr _0.2 O ₂ hardly undergo oxygen release reaction in air where the oxygen partial pressure is about 20%, but Ce _0.6 Zr _0.4 O _2. Then, an oxygen release amount of 1.08 g / cm ₃ was obtained.

Next, the ceramic sample S after completion of the oxygen release reaction was rapidly cooled, and an X-ray diffraction pattern was measured for each. The X-ray diffraction pattern after the oxygen release reaction is shown in FIG.
From the X-ray diffraction pattern shown in FIG.
That is, normally, when oxygen vacancies are generated, the peak of the X-ray diffraction pattern shifts to the low angle side. Further, when cation migration occurs and the ratio of Ce in the whole increases, the peak shifts to the low angle side. On the other hand, after the oxygen release reaction in Ar, both the cubic and tetragonal peaks are shifted to the low angle side, but after the oxygen release reaction in air, the cubic peak is shifted to the high angle side. . This is probably because cation migration occurred between the cubic and tetragonal crystals and the ratio of Ce in the whole changed. Ce _0.6 Zr _0.4 O ₂ is generally known to undergo a cubic-tetragonal phase transition around 1500 ° C. (Yashima, M. and Yoshimura, M., 1995: Phase Transition and Phase Diagram of Zrconia Ceramics, Materia Japan, 34 (4), 448-453.), The progress of this unique oxygen release reaction in the air leads to the generation of oxygen deficiency accompanied by the interaction between cubic and tetragonal crystals. Suggested to be due. In addition, the sample after the oxygen release reaction can undergo a water splitting reaction at around 500 ° C., and sintering does not proceed in the oxygen release process, so that continuous hydrogen production is possible.

(Comparative Example 1)
A ceramic sample in which Ce and Zr were dissolved at a molar ratio of 60:40 was prepared by the complex polymerization method as follows.
Cerium acetate _{(Ce (CH 3 COO) 3} · H 2 O, 3.516g) and zirconium oxychloride _{(ZrOCl 2 · 8H 2 O,} 2.44g) and the mixture solution was dissolved in distilled water 91.5Ml, citric This was mixed with an acid-ethylene glycol solution (ethylene glycol 28.0 ml, citric acid 22.49 g), and polymerized by heating at 400-600 ° C. Then, it heated at about 350 degreeC and the amorphous precursor was obtained. The precursor is heated to 1000 ° C. in air at a heating rate of about 10 ° C./min to be crystallized by heating, and further maintained at 1000 ° C. until the organic substances are completely removed. Thus, a ceramic sample was obtained.

  The ceramic sample prepared by this complex polymerization method was analyzed by an X-ray diffraction method, and it was confirmed that the ceramic sample was formed only by a tetragonal crystal system.

(Example 2)
Using the experimental apparatus 31 shown in FIG. 3, a two-stage water decomposition reaction experiment was performed on the Ce _0.6 Zr _0.4 O ₂ reactive ceramic sample obtained in Example 1.
The reactive ceramic sample Ce _0.6 Zr _0.4 O ₂ is placed on the platinum net 33 a of the reaction unit 33 of the experimental apparatus 1, Ar gas is supplied at 100 ml / min, and infrared rays are emitted from the infrared image furnace 34. The oxygen release reaction was performed by irradiation and heating at 1500 ° C. for 5 minutes. Next, a mixed gas of water vapor (450 ml / min) and Ar gas (100 ml / min) is supplied to the reaction section 33, and at 500 ° C., 1000 ° C., 800 ° C., 600 ° C., 400 ° C. and 500 ° C., 5 ° C., respectively. Hydrogen generation reaction was performed by heating for a minute. Then, the amounts of oxygen and hydrogen generated in each reaction were measured by the direct gas mass spectrometer 73 of the analysis unit 7. FIG. 5A shows the oxygen release amount in the oxygen release reaction at 1500 ° C. (1773 K) at this time, and FIG. 5B shows the hydrogen generation amount in the hydrogen generation reaction at 500 ° C. (773 K).

As shown in FIGS. 5A and 5B, in the reactive ceramic sample Ce _0.6 Zr _0.4 O ₂ mainly composed of a cubic crystal system, the hydrogen generation reaction proceeds very quickly. I understood. Further, the hydrogen generation amount shows a maximum value around 500 ° C. This was the same for the reaction ceramic samples CeO ₂ and Ce _0.8 Zr _0.2 O ₂ prepared in Example 1.

As described above, in the reactive ceramic sample Ce _0.6 Zr _0.4 O ₂ mainly composed of a cubic crystal system, the temperature at which the hydrogen generation amount shows the maximum value is around 500 ° C. It was found that hydrogen can be generated at a significantly lower temperature than the reactive ceramics (for example, ferrite-based hydrogen generation reaction temperature: 100 ° C. or higher).

(Comparative Example 2)
Using the reactive ceramic Ce _0.6 Zr _0.4 O ₂ formed only by the tetragonal crystal system prepared in Comparative Example 1, a two-stage water splitting reaction experiment was conducted in the same manner as in Example 2. .
In this experiment, the oxygen release reaction was performed at 1500 ° C. for 5 minutes, and the hydrogen generation reaction was performed at 500 ° C., 1000 ° C., 1200 ° C., 1300 ° C., and 1100 ° C. for 5 minutes after the oxygen release reaction. FIG. 6A shows the oxygen release amount in the oxygen release reaction at 1500 ° C. (1773 K), and FIG. 6B shows the hydrogen generation amount in the hydrogen generation reaction at 500 ° C. (773 K). However, since the water decomposition reaction of platinum proceeds at 1200 ° C., the hydrogen generation amount shown in FIG. 6B is the platinum water decomposition calculated stoichiometrically from the oxygen generation amount at the time of supplying steam. The value excluding the amount of hydrogen produced by the reaction is shown.

From the results shown in FIGS. 6 (a) and (b), the reactivity obtained in Comparative Example 1 was different from the case where the reactive ceramic Ce _0.6 Zr _0.4 O ₂ obtained in Example 1 was used. It can be seen that in ceramics, the hydrogen generation reaction does not occur at 500 ° C., but occurs at 1000 ° C. or higher. Further, the temperature at which the amount of hydrogen generation reaches the maximum value (optimum hydrogen generation reaction temperature) is around 1200 ° C. Furthermore, when compared with the reactive ceramic Ce _0.6 Zr _0.4 O ₂ obtained in Example 1, both the oxygen releasing reaction and the hydrogen generating reaction have a reaction rate other than the first oxygen releasing reaction. Slow and produces little oxygen.

From these results, the reactive ceramic Ce _0.6 Zr _0.4 O ₂ formed only by the tetragonal crystal system obtained by the complex polymerization method of Comparative Example 1 and the oxalic acid coprecipitation method of Example 1 were obtained. With the reactive ceramic sample Ce _0.6 Zr _0.4 O ₂ mainly composed of a cubic crystal system obtained in the above, the reactivity may be greatly different if the crystal system is different even with the same composition. I understood.

(Example 3)
Using the experimental apparatus 31, in the same manner as in Example 2, the reactive ceramics CeO ₂ , Ce _0.8 Zr _0.2 O ₂ and Ce _0.6 Zr _0.4 O ₂ obtained in Example 1 were used. Each was subjected to a two-stage water splitting reaction at different oxygen release reaction temperatures. The oxygen releasing reaction was performed twice at 1500 ° C., 1400 ° C., 1300 ° C., and 1200 ° C. for 5 minutes each (3 times only at 1500 ° C.) for 5 minutes. After completion of each oxygen releasing reaction, a hydrogen generation reaction was performed at 500 ° C. for 5 minutes. FIG. 6 shows the oxygen release reaction temperature dependence of the amount of hydrogen produced at this time.

As shown in FIG. 7, when the reactive ceramics CeO ₂ and Ce _0.8 Zr _0.2 O ₂ having almost the same crystal structure are compared, the oxygen release amount and the hydrogen generation at any oxygen release reaction temperature It can be seen that the amount of reactive ceramic Ce _0.6 Zr _0.4 O ₂ is larger.

(Example 3)
Using the experimental apparatus 31, the oxygen release reaction (1500 ° C., 5 ° C.) using the reactive ceramic Ce _0.6 Zr _0.4 O ₂ mainly composed of a cubic crystal system obtained in Example 1. Minute) and hydrogen generation reaction (500 ° C., 5 minutes) was repeated two times to perform a two-stage water splitting reaction. The oxygen release amount and the hydrogen generation amount at this time are shown in FIGS. 8A and 8B, respectively.

  As shown in FIGS. 8A and 8B, the first oxygen release amount is larger than the second and subsequent oxygen release amounts, and other oxygen release amounts and hydrogen generation amounts can be obtained even if the cycle is repeated. It turned out to be almost constant.

It is a schematic diagram which shows the specific example of a hydrogen production apparatus. 2 is a diagram showing an X-ray diffraction pattern of the reactive ceramic obtained in Example 1. FIG. It is the schematic which shows the experimental apparatus for performing oxygen releasing reaction and hydrogen production reaction. It is a figure which shows the X-ray-diffraction pattern after completion | finish of oxygen releasing reaction of the reactive ceramics obtained in Example 1. FIG. (A) is an oxygen release amount of the reactive ceramic prepared in Example 1, and (b) is a diagram showing a hydrogen generation amount. (A) is an oxygen release amount of the reactive ceramic prepared in Comparative Example 1, and (b) is a diagram showing a hydrogen generation amount. It is a figure which shows the oxygen release reaction temperature dependence of the production amount of hydrogen in a two-stage water splitting reaction about each of the reactive ceramics obtained in Example 1. Using the reactive ceramic Ce _0.6 Zr _0.4 O ₂ obtained in Example 1, the oxygen release amount and the hydrogen generation amount in the two-stage water splitting reaction in which the oxygen release reaction and the hydrogen generation reaction are alternately repeated five times. FIG. It is a conceptual diagram explaining manufacture of hydrogen by the two-stage water splitting reaction using reactive ceramics.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Hydrogen production apparatus 2 Rotating rotor 3 Reaction cell 3a, 3b Heat insulation material 3c Case 4 Reactive ceramics 5 Oxygen release cell 5a Quartz window for sunlight introduction 5b Window 5c Oxygen exit 5d Gas introduction port 6 Hydrogen production cell 6a Quartz window 6b Window 6c Hydrogen outlet 7 Outer cylindrical body 6d Water vapor inlet 7a, 7b Quartz window support 8a, 8b Quartz window support 31 Experimental apparatus 32 Reaction tube 33 Reaction section 33a Platinum net 34 Infrared image furnace 35a, 35b Pipe line 36 Water vapor generation apparatus 61 Water storage unit 62 Micro pump 63 Water vapor generation unit 64 Electric furnace 65a, 65b Gas inlet 37 Analysis unit 71 Primary cooling unit 72 Secondary cooling unit 73 Direct gas mass spectrometer 38 Thermocouple 39a, 39b Pipe switching valve 40 Gas Introduction

Claims (7)

  A reactive ceramic comprising a solid solution containing cerium and zirconium, wherein a cubic crystal system and a tetragonal crystal system coexist.   The reactive ceramic according to claim 1, wherein the cerium / zirconium molar ratio is 90/10 to 20/80.   The reactive ceramic according to claim 1, wherein the cerium / zirconium molar ratio is 80/20 to 50/50. Mixing a nitrate aqueous solution containing cerium nitrate and zirconium nitrate with an oxalic acid aqueous solution to form a precipitate;
Adjusting the pH of the suspension containing the precipitate to 0-7;
Firing the precipitate;
A process for producing reactive ceramics, comprising: A method for producing hydrogen using the reactive ceramic according to any one of claims 1 to 3,
An oxygen release reaction in which the reactive ceramic is irradiated with concentrated solar energy and heated to 1400 to 1800 ° C. in an air atmosphere to reduce the reactive ceramic and release oxygen;
A hydrogen generation reaction in which the reduced reactive ceramic is reacted with water at 300 to 1200 ° C. to oxidize the reactive ceramic to generate hydrogen;
A method for producing hydrogen, wherein the two-stage water splitting reaction is repeated. 6. The method for producing hydrogen according to claim 5, wherein an energy flux of the concentrated sunlight energy is 1000 to 3000 kW / m ² . A two-stage water-splitting reaction part in which the reactive ceramic according to any one of claims 1 to 6 is disposed;
A solar light supply unit for supplying solar energy to the two-stage water splitting reaction unit;
A water supply unit for supplying water to the two-stage water splitting reaction unit;
With
In the two-stage water splitting reaction section, solar energy is supplied from the solar power supply section and the reactive ceramic is heated to 1400 to 1800 ° C. in an air atmosphere to reduce the reactive ceramic and reduce oxygen. In addition, hydrogen is produced by repeating a two-stage water splitting reaction in which the reduced reactive ceramic is reacted with water at 300 to 1200 ° C. to oxidize the reduced reactive ceramic to produce hydrogen. A hydrogen production apparatus characterized by:

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